The present invention relates to anti-islanding in a distributed power system and, more particularly, system and method for protection of photovoltaic distributed power equipment and personnel during anti-islanding.
Utility networks provide an electrical power system to utility customers. The distribution of electric power from utility companies to customers utilizes a network of utility lines connected in a grid-like fashion, referred to as an electrical grid. The electrical grid may consist of many independent energy sources energizing the grid in addition to utility companies energizing the grid, with each independent energy source being referred to as a distributed power (DP) generation system. The modern utility network includes the utility power source, consumer loads, and the distributed power generation systems which also supply electrical power to the network. The number and types of distributed power generation systems is growing rapidly and can include photovoltaics, wind, hydro, fuel cells, storage systems such as battery, super-conducting flywheel, and capacitor types, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines. These distributed power generation systems are connected to the utility network such that they operate in parallel with the utility power sources.
One common problem faced by modern utility networks is the occurrence of islanding. Islanding is the condition where a distributed power generation system is severed from the utility network, but continues to supply power to portions of the utility network after the utility power supply is disconnected from those portions of the network. All photovoltaic systems must have anti islanding detection in order to comply with safety regulations. Otherwise the photovoltaic installation may shock or electrocute repairmen after the grid is shut down from the photovoltaic installation generating power as an island downstream. The island condition complicates the orderly reconnection of the utility network and poses a hazard also to equipment. Thus, it is important for an island condition to be detected and eliminated.
Several techniques have been proposed to guard against islanding. For example, one method involves the monitoring of auxiliary contacts on all circuit breakers of the utility system between its main source of generation and DP systems. The auxiliary contacts are monitored for a change of state which represents an open circuit breaker on the utility source. The utility circuit breaker is typically monitored and tripped by external protective relays. When a loss of utility is detected by the change in state of the auxiliary contact of a circuit breaker, a transferred trip scheme is employed to open the interconnection between the utility and the distributed power system. A transferred trip scheme uses the auxiliary contacts of the utility source being monitored. The auxiliary contacts are connected in parallel with other devices which can trigger the trip of the local interconnection breaker. When the auxiliary contacts change state, a trip is induced on the local interconnection breaker. This prevents an island condition from occurring. The drawback of such a method is that often the point of utility isolation (the point at which the utility circuit breaker opens) is of such a distance from the local distributed power system that running a contact status signal back to the local distributed power system control system is not practical.
Anti-islanding schemes presently used or proposed include passive schemes and active schemes. Passive schemes are based on local monitoring of the grid signals, such as under or over voltage, under or over frequency, rate of change of frequency, phase jump, or system harmonics, for example. Active schemes are based on active signal injection with monitoring of the resulting grid signals, such as impedance measurement for example, or active signal injection with active controls, such as active frequency shifting or active voltage shifting for example. With active schemes, some distortion may occur in the output current waveform, thereby resulting in a tradeoff between islanding detection time and waveform distortion, with faster detection typically resulting in higher total harmonic distortion.
A conventional installation of a solar distributed power system 10, including multiple solar panels 101, is illustrated in FIG. 1. Since the voltage provided by each individual solar panel 101 is low, several panels 101 are connected in series to form a string 103 of panels 101. For a large installation, when higher current is required, several strings 103 may be connected in parallel to form overall system 10. The interconnected solar panels 101 are mounted outdoors, and connected to a maximum power point tracking (MPPT) module 107 and then to an inverter 104. MPPT 107 is typically implemented as part of inverter 104 as shown in FIG. 1. The harvested power from DC sources 101 is delivered to inverter 104, which converts the direct-current (DC) into alternating-current (AC) having a desired voltage and frequency, which is usually 110V or 220V at 60 Hz, or 220V at 50 Hz. The AC current from inverter 104 may then be used for operating electric appliances or fed to the power grid.
As noted above, each solar panel 101 supplies relatively very low voltage and current. A problem facing the solar array designer is to produce a standard AC current at 120V or 220V root-mean-square (RMS) from a combination of the low voltages of the solar panels. The delivery of high power from a low voltage requires very high currents, which cause large conduction losses on the order of the second power of the current i2. Furthermore, a power inverter, such as inverter 104, which is used to convert DC current to AC current, is most efficient when its input voltage is slightly higher than its output RMS voltage multiplied by the square root of 2. Hence, in many applications, the power sources, such as solar panels 101, are combined in order to reach the correct voltage or current. A large number of panels 101 are connected into a string 103 and strings 103 are connected in parallel to power inverter 104. Panels 101 are connected in series in order to reach the minimal voltage required for inverter 104. Multiple strings 103 are connected in parallel into an array to supply higher current, so as to enable higher power output.
FIG. 1B illustrates one serial string 103 of DC sources, e.g., solar panels 101a-101d, connected to MPPT circuit 107 and inverter 104. The current versus voltage (IV) characteristics is plotted (110a-110d) to the left of each DC source 101. For each DC power source 101, the current decreases as the output voltage increases. At some voltage value, the current goes to zero, and in some applications the voltage value may assume a negative value, meaning that the source becomes a sink. Bypass diodes (not shown) are used to prevent the source from becoming a sink. The power output of each source 101, which is equal to the product of current and voltage (P=i*V), varies depending on the voltage drawn from the source. At a certain current and voltage, close to the falling off point of the current, the power reaches its maximum. It is desirable to operate a power generating cell at this maximum power point (MPP). The purpose of the MPPT is to find this point and operate the system at this point so as to draw the maximum power from the sources.
In a typical, conventional solar panel array, different algorithms and techniques are used to optimize the integrated power output of system 10 using MPPT module 107. MPPT module 107 receives the current extracted from all of solar panels 101 together and tracks the maximum power point for this current to provide the maximum average power such that if more current is extracted, the average voltage from the panels starts to drop, thus lowering the harvested power. MPPT module 107 maintains a current that yields the maximum average power from system 10.
However, since power sources 101a-101d are connected in series to single MPPT 107, MPPT 107 selects a maximum power point which is some average of the maximum power points of the individual serially connected sources 101. In practice, it is very likely that MPPT 107 would operate at an I-V point that is optimum for only a few or none of sources 101. In the example of FIG. 1B, the selected point is the maximum power point for source 101b, but is off the maximum power point for sources 101a, 101c and 101d. Consequently, the arrangement is not operated at best achievable efficiency.
The present applicant has disclosed in co-pending U.S. application Ser. No. 11/950,271 entitled “Distributed Power Harvesting Systems Using DC Power Sources”, the use of an electrical power converter, e.g. DC-to-DC converter, coupled to the output of each power source, e.g. photovoltaic panel. The electrical power converter converts input power to output power by monitoring and controlling the input power at a maximum power level. This system may be used also to address the anti-islanding issue.
The term “leakage” as used herein refers to electrical power which is radiated or conducted into an electrical signal line typically at low levels and typically because of insufficient isolation.